1. Field of the Invention
The present invention relates generally to a process for forming siloxane polymers and, more particularly, to a photochemical vapor deposition process for depositing layers of such polymers on a substrate.
2. Description of the Related Art
Dielectric or insulating materials such as silicon dioxide and silicon nitride are widely used in the fabrication of semiconductor devices and circuits to provide a layer of electrical insulation which prevents contact or unwanted current flow between adjacent conductive areas or materials. In addition, such insulating materials are used as a surface passivation layer to protect the surface of the substrate or as a mask during such procedures as etching or ion implantation. Such layers have typically been formed by thermal chemical vapor deposition at elevated temperatures, by plasma-enhanced chemical vapor deposition, or by reactive sputtering or nonreactive sputtering techniques. However, the latter three processes have the disadvantage of producing charge bombardment or radiation bombardment of the substrate and consequent degradation of device performance, while the thermal process produces thermal damage to the substrate. A recently developed process for oxide deposition which overcomes these disadvantages is the photochemical vapor deposition process described in U.S. Pat. No. 4,371,587, assigned to the present assignee. Similar photochemical processes for depositing nitride layers and sulfide layers are described in U.S. Pat. Nos. 4,181,751 and 4,447,469 respectively, both assigned to the present assignee.
As an alternative to oxide or nitride passivation layers, certain polymers have been used for this purpose. One such polymer is Parylene (a para-xylylene available from Union Carbide) which has been produced by vapor phase deposition and polymerization on the surface of the substrate. However, there is sometimes a problem with achieving good adhesion of the Parylene to the substrate. In other cases, it is sometimes difficult to remove the Parylene layer for repair of the underlying device.
Another polymer which is useful as a passivation layer for microelectronic devices and circuits is a silicone or polymer of siloxane compounds of the formula R.sub.2 SiO, when R is an alkyl group. A known method for forming a polysiloxane encapsulant is described, for example, in U.S. Pat. No. 4,163,072 to Soos and comprises applying a coating of an uncured room temperature vulcanizing (RTV) silicone on the substrate and exposing the RTV silicone to moisture at a temperature of 90.degree.-130.degree. C. and for a time (e.g. 1 to 24 hours) sufficient to cure the silicone.
While the above-noted polymers have been useful for some purposes, the need exists for a process for depositing an adhesive layer of a siloxane protective polymer on a substrate in the fabrication of semiconductor devices and circuits or other structures in such a manner that the properties of the siloxane polymer layer can be tailored to meet particular requirements.